The invention relates generally to solid-state image sensing apparatus and, more particularly, to image sensing apparatus including a readout circuit that compresses high image intensity signal levels.
Solid-state image sensing apparatus is well known. Such sensing apparatus includes a plurality of photosensitive image detector elements. In accordance with well known principles, a charge is generated within each detector element as a function of the amount of electromagnetic radiation that is incident thereon. The plurality of detector elements are typically integrated in a semiconductor substrate as either a linear image sensor or as an area image sensor. The linear image sensor includes a single line of detector elements that can be used to monitor a one-dimensional variable, e.g., liquid level, or that can be used to obtain a two-dimensional image by mechanically scanning the object or scene to be imaged, such as by use of a rotating mirror. The area image sensor, also known in the art as a staring array sensor, is a two-dimensional array of detector elements onto which is projected an image of the object or scene to be imaged.
The composition of the detector elements of the image sensor is determined, in part, by the wavelength of radiation being imaged. For example, in the case of an image sensor for imaging infrared (IR) radiation, the detecting elements can be constructed as Pd.sub.2 Si or PtSi Schottky barrier detectors. An example of image sensing apparatus including PtSi detector elements, and a method for making same, is described in "160.times.244 Element PtSi Schottky-barrier IR-CCD Image Sensor" by Kosonocky et al., IEEE Transactions on Electron Devices, Vol. ED-32, No. 8, August 1985, which is incorporated herein by reference. The detecting elements of a sensor for imaging visible light can, for example, be constructed as a silicon photovoltaic diode.
Solid-state image sensing apparatus also includes solid-state integrated circuitry for reading out signals representative of the image detected by the detector elements. Such readout circuitry can be integrated on the same semiconductors substrate with the detector elements to provide monolithic image sensing apparatus. Alternatively, the readout circuitry can be integrated on a separate substrate which is interconnected with the substrate containing the detector elements to provide hybrid image sensing apparatus. The readout circuitry typically includes a configuration of charge coupled devices (CCD) or switched capacitor circuitry (SCC), although other types of circuitry are known in the art for transporting the charge generated by each detector element to an external image readout terminal. Solid-state image sensing apparatus also includes a region associated with each detector element, for storing the charge generated by that detector element in response to incident electromagnetic radiation. In accordance with known designs of image sensing apparatus, the location of the charge storing region varies with the design of the apparatus. Such locations can include a portion of the structure of the detector element or a region of the substrate in which the readout circuitry is integrated.
In operation, the detector elements of the image sensing apparatus generate charge in response to incident electromagnetic radiation and the generated charge is stored, over a predetermined charge integration period, in the charge storing region. Then, the stored charge is transferred through the readout circuitry to transport the charge associated with each detector element to the external image readout terminal of the imaging apparatus.
Solid-state image sensing apparatus such as described above also frequently includes blooming control circuitry. As is well known in the art, blooming is an overload condition that results from imaging excessively "bright" objects, i.e., objects having a high image intensity. The blooming corresponds to a spreading of the large amount of charge generated by a detector element in response to the high image intensity, such spreading potentially emanating from the charge storing region and/or the readout circuitry, e.g., in the CCDs.
Various solutions are known for minimizing the adverse effects of blooming. One known solution is to drain off the excessive charge, generated by a detector element, that exceeds a maximum charge storage capacity. Disadvantageously, such a solution results in the loss of signal level information for the bright object. Another known solution is to adjust the operation of the image sensing apparatus, such as by providing a shorter image charge integration period, to enable clear imaging of bright objects. Unfortunately, this solution results in a loss of image information corresponding to "dim" objects, i.e., objects having a low image intensity. As used herein, the terms "bright" and "dim" do not refer to a particular portion of the electromagnetic spectrum and are only intended to connote high and low image intensity, respectively.
Attempted solutions to the above described problems of blooming control should preferably require a minimum amount of additional circuit area on the substrate in which the image sensing apparatus is integrated. Also, attempted solutions should preferably not adversely effect the rate at which image information is readout from the image sensing apparatus. Further, attempted solutions should preferably not adversely effect a signal-to-noise ratio associated with the image signals generated for dim objects.
It is therefore an object of the present invention to provide solid-state image sensing apparatus that includes means for imaging objects having a high intensity that is not subject to the aforementioned problems and disadvantages of the prior art.
It is another object of the present invention to provide solid-state image sensing apparatus which does not require significant area on a substrate in which the image sensing apparatus is integrated.
It is a further object of the present invention to provide solid-state image sensing apparatus which does not adversely effect an image readout rate.
It is an additional object of the present invention to provide solid-state image sensing apparatus which is compatible with different types of image information readout circuitry.